Uranium, a naturally occurring element in earth's crust, exists in soil, sediments, plants, and river water, and particularly, it is known that 0.76 μg of uranium per 1 g of soil is present in the soil. Among uranium isotopes, 238U (99.2745%) is a dominant uranium isotope. In addition, 235U and 234U are also considered as major isotopes. All of them are classified as a radioactive material emitting α-ray and having a long half-life. Use of uranium, which is an actinide element, has increased; it is used as a raw material required for manufacturing a new kind of chemical species or alloys due to its own physical/chemical properties and as nuclear fuel in nuclear power generation, or the like.
A quantitative analytical technology for uranium is essential for controlling processes related to mining, purification, processing, separation, and recovery of uranium. Particularly, analytical techniques of uranium related to environmental health have been developed extensively. For example, in order to study the effects of uranium variability in river water according to geological conditions or artificially released uranium on the ecosystem, or the like, or in order to track migration pathways of uranium in groundwater systems, developments of appropriate analytical methods have been required for measuring a concentration of a trace amount of uranium in samples. Currently, for uranium analysis, α-spectroscopy, laser luminescence spectroscopy, inductively coupled plasma (ICP)-mass spectroscopy (MS), or the like, is mainly used. Among them, a spectroscopic technique using luminescence properties of uranium is a highly sensitive and non-destructive method as compared with the alpha spectroscopy.
The laser induced luminescence spectroscopy may be divided into a continuous wave laser spectroscopy and a pulse laser spectroscopy according to the type of laser source. Use of continuous wave laser may be advantageous to obtain strong luminescent signal due to the accumulation of continuous luminescence from uranium species in a sample. A detector system may be comprised of a fiber optic light guide, a monochromator, and a photo-multiplier tube, or the like, which thereby allows a highly sensitive luminescence measurement. In the case in which luminescence properties of chemical species to be measured are well-defined under controlled sample conditions (pH, concentration, ionic strength, temperature, or the like), direct measurements of luminescence intensity are simple and advantageous ways for uranium quantification. Particularly, luminescence intensity at a wavelength range of 508 to 525 nm where a strong luminescence peak lies can be directly used for the uranium quantification. On the other hand, a measurement system using the pulse laser system may be more complicated (See below). However, attenuation waveforms of luminescent signal after each pulse can be measured separately in addition to luminescence intensity. Using such luminescence properties, i.e., lifetime and spectrum peak positions, more selective identification of each uranium species can be achieved; a luminescent signal from a single species may be measured and distinguished even though a sample contains a mixture of uranium chemical species. Such a pulse laser-based luminescence technique is typically called time-resolved laser-induced fluorescence spectroscopy (TRLFS), and a detailed description is provided below.
Uranium(VI) (oxidation number: 6+) exhibits strong emission when the pulse laser is irradiated. This luminescence (LM) emission has been used to selectively distinguish various uranium species in a sample since each chemical species has a unique luminescence spectrum and luminescence lifetime. For example, a luminescence lifetime of uranyl ions (UO22+) in a strong acidic aqueous solution is about 1 to 2 μs, but luminescence lifetimes of (UO2)2(OH)22+ and (UO2)3(OH)5+, which are hydrolyzed species present in weak acidic aqueous solution, are 6 to 8 μs and 10 to 15 μs, respectively, so that they can be distinguished from each other. In addition, the luminescence spectrum of each species also has different peak positions from each other, that is, a unique spectral signature of each U(VI) species. In addition, when an organic or inorganic ligand is present in aqueous solution, uranium ions form metal-ligand complexes, wherein luminescence properties of these complex species are sensitively changed according to a composition of the aqueous solution. Therefore, these luminescence properties are useful to track physicochemical behaviors of uranium ions in the aqueous solution, and a description thereof is disclosed in Korean Patent Laid-Open Publication No. 10-2012-0079941 (Patent Document 1).
In order to measure both luminescence intensity and luminescence lifetime of uranium(VI), a pulse laser (light source), a monochromator, and a detector such as a photo-multiplier tube (PMT) detector are required. Generally, a laser source having an excitation wavelength of 420 nm or less and a pulse width of several nanometers is used. Particularly, in order to measure the luminescence lifetime, devices such as an oscilloscope capable of measuring the attenuation of luminescence signals after irradiation of a laser pulse and a boxcar (a time-gate controller and signal averager) for tracking the attenuated luminescence signal at each time point are required. In TRLFS, a set of these devices is typically used to simultaneously measure luminescence intensity and luminescence attenuation as well as luminescence spectrum by controlling the width of a detection gate and delay time at a level of several microseconds (μs) or less. In the case in which each chemical species, i.e., each U(VI) species, has different luminescence spectrum and lifetime, the TRLFS is a significantly effective method in distinguishing chemical species. Generally, the TRLFS is highly useful to detect and analyze uranium chemical species in an aqueous solution, which generally have short luminescence lifetimes (several μs or less).
In the case in which phosphoric acid or a polymeric form thereof is present, uranium forms UO2-phosphate complexes, wherein a luminescence lifetime of this complex species is significantly longer than that of other uranium chemical species. That is, when a laser pulse having a wavelength of 425 nm or less is irradiated under an acidic condition, generally, luminescence emission exhibits a long lifetime of 50 to 400 μs. Based on such luminescence characteristics of this complex, a kinetic phosphorescence analysis (KPA) method was developed for determination of uranium, which in fact is one type of TRLFS. This property, i.e., the extended lifetime of U(VI), contributes to an increase in the measured luminescence intensity of uranium. In the KPA method, after the irradiation of each laser pulse, the attenuation of light emitted at a wavelength of 515 to 520 nm is measured using a multi-channel counter. Then, the luminescence decay profile is analyzed based on the principle that the y-intercept value obtained by extrapolating a plot of log values of the measured luminescence intensity as a function of time is in proportion to a concentration of uranium. Along with sample pre-treatment procedures performed prior to the spectroscopic measurement, the KPA method is a highly sensitive analytical method enough to have a limit of detection of several ten ng/L (<nM).
However, since the luminescence phenomenon of uranium as described above is observed only for uranium species in an oxidation state, 6+; uranium in other oxidation states, such as +4 and +3, are generally known to be non-luminescent. If uranium having a different oxidation state rather than 6+ is present in a sample, determination of the total uranium may be impossible. In addition, when other metal ions or inorganic/organic materials quenching uranium luminescence coexist in the sample, the luminescence lifetime and intensity may be significantly reduced. As shown in FIG. 1, although a sample containing uranium is mixed with the luminescence enhancing phosphate as mentioned above, the luminescence lifetime of uranium rapidly decreases as the concentration of reductive metal ions or organic/inorganic material increase. Therefore, prior to spectroscopic measurements, a sample pre-treatment process for separating or decomposing these interfering materials is required. The pre-treatment process is generally configured of a wet ashing process requiring high temperature (400 to 600° C.) and strong acid, and a dry ashing process. The object of the pre-treatment process is to convert uranium in lower oxidation states other than 6+ to uranium of +6 oxidation state through oxidation reactions and to reduce interfering materials by decomposing/evaporating the organic or inorganic materials. However, even after this pre-treatment process is performed, it is known that residual metal ions or other organic or inorganic ions can affect analysis sensitivity (See FIG. 1). Further, a series of such complicated and labor-intensive pre-treatment procedures increases uncertainty of the analysis and makes it difficult to implement a rapid uranium analysis. Therefore, the necessity for a more simple and accurate method of detecting a concentration of uranium has increased.